Top quarks in Run 2 are spot on

This new result at 13 TeV (red circle) is compared to previous results from ATLAS using the same analysis techniques (blue points) and the most precise measurement from the Tevatron (star). This shows the excellent agreement with QCD theory over a wide range of energies. (Image: CERN)

With a precision of just under 14% − currently dominated by our ability to understand how many proton-proton collisions have occurred at ATLAS (i.e. luminosity) − this measurement is able to confirm that quantum chromodynamics, the theory of the strong interaction, still seems to be going strong!

Being able to make this measurement in such a short period of time is a true testament to the quality of the data taken so far and to the level of understanding of our detectors achieved in only a few weeks.

The analysis technique used is very similar to that used in the most precise measurements to date (see Run I search for new massive bosons builds excitement for Run II). By using events with a >88% purity in top quark pairs, and using a simple method of counting events identified as having one electron, one muon and either one or two jets likely to originate from b-quarks, this analysis is able to make a significant statement about the validity of the Standard Model at the highest energies ever achieved.

The cross section (which corresponds to a probability) for top quark production at 13 TeV is measured to be 825 ± 113 pb, 3.4 times the cross section at 8 TeV. Being able to make this measurement in such a short period of time is a true testament to the quality of the data taken so far and to the level of understanding of our detectors achieved in only a few weeks.

As the top quark is believed by many to play a special role in our quest for new physics, the cross section measurement’s compatibility with the prediction could thus be seen as disappointing. But we are still a long way from the 4% precision of the Run 1 measurement, leaving plenty of room for new physics in this early measurement. Also, with more data we will be able to start probing differential distributions, looking in particular in the tails of high momentum top quark pairs. We hope to find new physics there.